Joan Lunden has breast cancer, is in 'warrior mode'

The journalist, author, health advocate and former "Good Morning America" cohost pens up about her diagnosis.

The journalist, author, health advocate and former "Good Morning America" cohost pens up about her diagnosis.

Christie D'Zurilla

Joan Lunden is undergoing chemotherapy for breast cancer and will follow up with a lumpectomy and radiation

Joan Lunden, who co-hosted "Good Morning America" for 17 years, has been diagnosed with breast cancer.

"In the beginning, it's such a shock, it's a stunner, and you think, 'I do everything right!,'" she said on her old show, sitting down Tuesday to break the news with current co-host Robin Roberts, a breast-cancer survivor.

Lunden has already started chemotherapy, she said in a statement, "and while I don’t know how I’ll look and feel down the road, I am assuming the best because that is how I live my life." In addition to chemo, she'll undergo a lumpectomy and radiation, her rep said, and is expected to make a full recovery.

"In the beginning you almost feel like, 'What did I do wrong?,'" Lunden told Roberts. "Then I just turned a corner and said, wait a second ... just, like, knock this off, learn everything you can about this, and go into warrior mode."

The 63-year-old said she's gotten mammograms "religiously," as well as ultrasounds because she has dense breasts. Nothing showed in this year's mammogram, she said, but the ultrasound was a different story.

The technician "kept going back to one spot, and I said, 'They found something.'"

As soon as the doctor walked into the post-biopsy follow-up appointment, her demeanor told the story, Lunden said, but still, "The words are almost surreal, when you hear, 'You have breast cancer.'"

On her blog, the mother of seven explained her decision to go public. "[S]ince the moment I took the job at Good Morning America I have lived my life sharing my joys and my disappointments with all of you: my pregnancies, my relationships, my career … I have shared my journey. So it certainly didn’t feel right keeping this part of my journey a secret."

Even with her background as a journalist and health advocate, Lunden said, it took "some time" to get comfortable with the notion of telling the world.

"Thank you to everyone who is reaching out to me with love & support," she wrote on Facebook. "It is an overwhelming feeling to hear from all of you and knowing you are with me on my journey brings me strength."